1. Field of the Invention
The present invention relates to an optical recording method which enables recording in a phase-change optical recording medium with a wide range of recording linear velocity, a phase-change optical recording medium and an optical recording apparatus which are used for the optical recording method.
2. Description of the Related Art
A phase-change optical recording medium (hereinafter referred to as ‘phase-change optical information recording medium’, ‘optical recording medium’, ‘optical information recording medium’, and ‘information recording medium’) is applied in a high-density and high-capacity optical recording medium having a capacity of 20 GB or greater which uses a CD, a DVD and moreover an LD (laser diode) having a wavelength of 400 nm. Technology to record at further higher speed has been desired particularly for the CD and the DVD as a rewritable optical recording medium. The speed has been considerably increased for CDs, and at present there is an increasing demand for the speeding up for rewritable DVDs. Among the DVDs, 2.4× speed has already been put into practical use for DVD+RW medium, which is one type of the rewritable DVDs; recording linear velocity of 4× speed, further 8× speed and faster is currently demanded.
In this case, it becomes a problem to what extent optical recording apparatuses coming in the market corresponds to the speeding up even though the phase-change optical recording medium can be used practically for the high speed recording. The number of rotations of a spindle motor for rotating the optical recording medium in the optical recording apparatus to perform a high speed recording is increasing and has reached 10,000 rotations (rpm) or more. Therefore, it is necessary to reduce disc surface deviation, disc eccentricity and warp of the optical recording medium by performing even more stable rotational control. Moreover, improvement in a servo control of a pick up for focusing and stably tracking grooves in which recording is to be done has been required.
However, in the current situation, there are limitations on these functions, and particularly the number of rotations of an inner circumference side of the phase-change optical recording medium cannot be increased to be more than required for a high-speed recording. Therefore, in order to perform a high-speed recording, a so-called CAV (Constant Angular Velocity: the number of rotations is constant) recording method has been adopted, where the recording takes place at a maximum recording linear velocity on the outer circumference side. In this CAV recording method, since the recording linear velocity changes according to a recording position in the same optical recording medium, it is necessary that the recording can be performed with favorable recording characteristics as well as uniformly.
When the recording linear velocity exceeds 10× speed and approached even to a higher velocity such as 16×, the CAV recording becomes difficult because the linear velocity goes on increasing continuously,. Optimum recording conditions of the phase-change optical recording medium can be found for any linear velocity at high speed. However, the pulse of a laser driving circuit in the optical recording apparatus becomes difficult to control while recording by varying the optimum recording conditions continuously. Therefore, in the high speed recording, a recording with a CLV (Constant Linear Velocity: linear velocity is constant) recording method at a constant linear velocity is more reliable. However, when the CLV recording is used and an attempt is made to record information of a certain address, a load of changing the number of rotations increases according to a position. Therefore, it can be taken into consideration that a ZCLV method may be adopted instead of the CAV recording and the CLV recording, where the linear velocity is set at a constant value for each of the several zones of the phase-change optical recording medium. In any case, it is necessary to have favorable recording characteristics and to record uniformly at each recording linear velocity.
On the other hand, an optimization of the phase-change optical recording medium is necessary for enabling the high speed recording. To correspond to a wide range of recording linear velocity as mentioned above, it is necessary to use a phase change recording layer material with which mark formation is easy at any of a high recording linear velocity and a low recording linear velocity, and furthermore, it is necessary to use a favorable erasing ratio for improving overwriting characteristics. However, when the phase-change optical recording medium is adjusted such that the characteristics are achieved at the maximum recording linear velocity, for not to lose the repeated overwriting properties, a phase-change recording layer with a material and composition having high erasability, in other words, high crystallization speed at a high recording linear velocity, are to be used.
Moreover, while forming a mark at a low recording linear velocity, a high write power is required for narrowing the width of an optical pulse which applies a write power. Furthermore, it is necessary to form the mark by irradiating a high write power at the maximum recording linear velocity since the crystallization speed is high and the temperature of the phase-change recording layer is not prone to rise.
To solve such issues, Japanese Patent Application Laid-Open (JP-A) No. 2001-118245 discloses, for example, in the CAV recording method in which the optical recording medium is rotated at a constant angular velocity and the recording linear velocity increases from the inner circumference to the outer circumference, a laser beam irradiated to the optical recording medium is controlled by three values, namely power corresponding to a heating pulse and a cooling pulse and an erase power, and the width of the heating pulse in a pulse string, which is comprised of the heating pulse and the cooling pulse, and the duty ratio of the final pulse section of the cooling pulse are varied according to the recording linear velocity. Moreover, the literature discloses a recording method to achieve uniform and favorable characteristics from the inner circumference to the outer circumference by recording while varying the ratio of the write power (Pw) and the erase power (Pe) Pe/Pw at a predetermined interval.
Moreover, among the CAV recordings in which the recording linear velocity of the optical recording medium increases from the inner circumference to the outer circumference, JP-A No. 2003-019868 proposes a recording method regarding a ZCAV method, which divides the medium into multiple zones according to a certain radius regions, where the write power irradiation time of a pulse string of the irradiated light, the bias power irradiation time, the write power, the ratio of the erase power and the write power and the bias power are set lo variable according to the zone, and at least the write power irradiation time is to be reduced monotonously from the inner circumference to the outer circumference.
Moreover, DVD+RW 4.7 GB Basic Format Specifications version 1.2′ (1×-4× DVD+RW standards) adopts the CAV/CLV recording methods, and it discloses that, in the CAV recording method, the optimum recording conditions (optical pulse control parameters and Pe/Pw ratio) are imparted to each of three linear velocities, namely the minimum recording linear velocity, the maximum recording linear velocity and a mean recording linear velocity which is halfway between the minimum recording linear velocity and the maximum recording linear velocity in each of the recording linear velocity ranges, 1.65× to 4× and 1× to 2.4×.
However, none of the Japanese Patent Application Laid-Open (JP-A) No. 2001-118245 and JP-A No. 2003-019868 as well as DVD+RW 4.7 GB Basic Format Specifications version 1.2′ (1×-4× DVD+RW standards) discloses or suggests findings related to a peculiar recording linear velocity while performing the recording with the maximum speed and the solutions thereof, and in the current situation further improvement and development is expected.